This invention relates to an image forming apparatus, such as a printer, facsimile, copier, and the like, that forms images by use of an electrophotographic method to develop electrostatic latent images with colored particles (toner), and to a method for detecting the detection characteristics of an optical reflection density sensor which is used in the image forming apparatus.
In the electrophotographic image forming field of copiers and laser printers, to stabilize the supply of toner to developers (replacement of developers) and the formation of images, the image forming apparatus forms a reference test pattern for controlling the image forming conditions on a photosensitive element or an intermediate transfer element under a preset operating condition, detects the quantity of toner on the test pattern, controls the supply of toner to the developer or the image forming condition (such as the charging potential, the exposure intensity, and the developing bias on the photosensitive element) and thus controls the quality of the recorded image.
In the steps of detecting the quantity of toner on the test pattern and controlling the image forming conditions, it is important that the detection characteristics of the optical reflection density sensor are stable with time. However, the detection characteristics of the optical reflection density sensor in actual use tend to deteriorate because of time-lapse deterioration of the light emitting diode (LED) that is used as is a light source for illuminating the test pattern and the light receiving photo diode (PD) that is used as a detection element and because of time-lapse contamination of the optical system. To suppress this influence, the detection characteristics of the optical reflection density sensor must be detected before the quantity of attached toner is detected.
One of the known methods for detecting the detection characteristics of the optical reflection density sensor is disclosed in Japanese Application Patent Laid-open Publication Hei 07-225501. Using a calibration reflector as a reference, this method detects the intensity of light reflected by the reflector and effects control to keep the result of detection at a preset value.
Another known detecting method is disclosed in Japanese Application Patent Laid-open Publication Hei 01-197777. Using a reflection density sensor having a characteristic in which the detection output is reduced as more toner attaches, this method forms a high-density test pattern toner image containing 1.5 to 3 times the usual quantity of toner attachment on the toner retainer by increasing the developing bias, detects the quantity of toner attached to the test pattern toner image and corrects the detection output.
Among the conventional methods for detecting the detection characteristics of an optical reflection density sensor, the detection method using a reference calibration reflector requires a driving mechanism for retracting the calibration reflector when the characteristic detection is not being implemented. This increases the required amount of mounting space and the number of parts. Therefore, it is hard to apply this method to a small inexpensive image forming apparatus.
Further, the method which uses a high-density test pattern toner image cannot detect such a high-density test pattern toner image when the high-density test pattern toner image cannot be formed by increasing the developing bias. This decreases the control precision. As one of the reasons why a high-density test pattern image cannot be formed, we can assume that this is due to deterioration of the developing ability of the developers that form the test pattern toner image.